1. Field of the Invention
This invention relates generally to an apparatus, such as an optical disc player, in which a light beam is directed against a record medium surface for optically reading information recorded in a track on such surface, and more particularly is directed to an optical scanning assembly for such apparatus.
2. Description of the Prior Art
It is known to provide an optical disc player with a focusing servo by which the light beam from a laser light source is correctly focused on the disc surface through an objective lens, and also with a tracking servo by which the point of impingement of the focused light beam on the disc surface is made to coincide with the track being scanned or read. In one known form of such optical disc player, the objective lens is mounted in a tubular lens holder which is guided for movement in the axial direction in response to electromagnetically applied forces for achieving the focusing servo function, and a galvano-mirror is interposed in the optical path of the light beam and is pivotally mounted on a rubber or other elastic support so as to be angularly displaceable by electromagnetically applied forces for achieving the tracking servo function. In such known arrangement, the focusing servo and tracking servo functions are performed with independent components, that is, the axially movable objective lens and the pivoted galvano-mirror, respectively, so that it is difficult to embody the same in a truly compact optical scanning assembly and to achieve reliable optical alignment of the various components. Further, the angular displacement of the galvano-mirror for effecting correction of tracking errors causes the displaced light beam to be incident on the objective lens at an angle to the optical axis, and this requires the use of a relatively expensive lens for correcting aberrations over a relatively wide range. Furthermore, in the case where rubber is used as the elastic support for the galvano-mirror, aging of the rubber will cause the characteristics of the support to vary with time and thereby vary the tracking servo performance.
In another known arrangement for performing the focusing servo and tracking servo functions, the objective lens is mounted in a tubular body or holder which is supported by pairs of leaf springs so that the objective lens can be displaced, in response to suitably applied electromagnetic forces, in respective directions substantially parallel to the optical axis of the objective lens and substantially at right angles thereto for achieving the focusing servo and the tracking servo functions, respectively. However, in such arrangement, when a tracking correction is effected, the optical axis of the objective lens may deviate angularly from its desired normal relationship to the surface of the record disc. Therefore, once again, the light beam may be incident on the objective lens at an angle to the optical axis thereof and it is necessary to use a relatively expensive objective lens corrected for aberrations over a relatively wide range away from the optical axis. Further, since the holder for the objective lens is supported by springs, resonance may result and, therefore, it is difficult to obtain reliable servo action over a wide range.